Spray-freezing apparatus and method

ABSTRACT

An apparatus and method are disclosed for forming fine frozen particles of a solution by using a refrigerated liquid coolant in which the solution is insoluble and the coolant is continuously supplied to a downwardly sloping plate to form a thin film of coolant on a surface of the plate and a nozzle is positioned to spray droplets of the solution against the thin film of coolant whereby the spray droplets of said solution are frozen by the coolant.

United States Patent [19] Murphy, Jr. et al.

SPRAY-FREEZING APPARATUS AND METHOD Inventors: James L.- Murphy, Jr.; Garland K.

Grace; Rudy E. Rogers, all of Huntsville, Ala.

Thiokol Chemical Corporation, Bristol, Pa.

Filed: May 25, 1971 Appl. No.: 146,651

Assignee:

US. Cl 62/347, 62/74, 264/ 14 Int. Cl. F25c l/00 Field of Search. 62/123, 74, 347; 34/5; 264/14 References Cited UNITED STATES PATENTS 3/1956 Kassel 264/14 Jan. 29, 1974 l-lallie et al. 264/14 x 3,092,553 6/1963 Fisher, Jr. et al.. 264/14 X 3,171,266 3/1965 Weiss 62/348 X 3,228,838 l/1966 Rinfret et al... 62/74 UX 3,294,672 12/1966 Torobin 62/ 123 X Prim'ary ExaminerWilliam E. Wayner 7] ABSTRACT An apparatus and method are disclosed for forming fine frozen particles of a solution by using a refrigerated liquid coolant in which the solution is insoluble and the coolant is'continuously supplied to a downwardly sloping plate to form a thin film of coolant on a surface of the plate and a nozzle is positioned to spray droplets of the solution against the thin film of coolant whereby the spray droplets of said solution are frozen by the coolant.

10 Claims, 5 Drawing Figures PATENTEDJAI 29 1974 SHE! 1 0f 3 INVENTORS. JAMES L. MURPHY, Jn GARLAND K. GRACE RUDY E. ROGERS PATENTEDJANZQISM SHEET 2 OF 3 F/GZ FIG. 3

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sum 3 UP 3 HHHUH l m M W: 4 n w it. NW1 2 u 3 v 6 mm W m 4 x I 7 n 2 6 5 SPRAY-FREEZING APPARATUS AND METHOD The present invention relates to the production of fine particles of solid materials by freeze-drying and, more particularly, to an improved apparatus and method for spray-freezing of droplets of a solution as a step in the production of submicron particles of the solute of the solution. The present method and apparatus have been found especially useful in the production of submicron size particles of ammonium perchlorate adapted to be used in th manufacture of solid propellants and will be illustratively described as used for this application. However, as the description proceeds, it will be apparent that the method and apparatus of the invention can equally apply to the production of ultrafine particles of other materials.

The conventional method of producing fine particles of ammonium perchlorate for use in propellants involves the mechanical grinding of ammonium perchlorate powders in various types of grinders, e.g., ball mills. Such mechanical grinding processes are subject to a number of disadvantages. Thus since a certain amount of heat is created by friction during such' grinding processes, a serious hazard of explosion exists. Morever, there is a certain amount of abrasion of the grinding surfaces which introduces undesirable impurities into the material being ground. Also it generally has not been possible by conventional grinding methods to achieve the submicron particle sizes that am: desirable for propellant applications.

Because of the disadvantages of the mechanical grinding methods, a number of alternative processes have been proposed. Among these alternatives is a freeze-drying process which involves, as a first step, preparing a solution of ammonium perchlorate or another solute, which is desired in small particle size, and spraying the solution into an appropriately refrigerated apparatus where it is frozen rapidly into small droplets. The droplets are then subjected to vacuum sublimation to evaporate the solvent and recover the solute in the form of small particles. A discussion of a freeze-drying method of this general type is contained in an article entitled; Production of Submicronic Powder by Spray-Freezing by Werly and Bauman contained in the November, 1964 issue of Archives ofEnvironmental Health.

The fineness of the particles obtained in such a process depends upon the manner in which the freezing step is carried out, i.e., upon such factors as the spray droplet size, the nature of the refrigerated environment into which the solution is sprayed'and the rate at which the solution droplets are frozen. As the droplets are frozen, .the solute tends to crystallize out and if a slow freezing rate is used, the particles of solute tend to agglomerate within the frozen droplet. However, if freezing is sufflciently rapid and the proper type of refrigerated medium is used, the solute particles formed are separated by ice crystals and do not have an opportunity to agglomerate.

It is evident, that the sprayed droplets should be as fine as possible to insure that each solution droplet is uniformly and rapidly frozen. To attain this result, it is necessary to utilize a properly designed spray apparatus that takes into account the various controlling characteristics of the nozzle, such as internal shape and orifice size. However, assuming that a properly designed nozzle is used, the nature of the refrigerated environment into which the droplets are sprayed requires careful selection and control in order to achieve the desired small particle size of the solute.

It is accordingly a general object of the invention to provide an improved method and apparatus for sprayfreezing droplets of a solution as a step in a freezedrying process of the type described above for producing submicron particles of the solute. It is another object of the invention to provide a method and apparatus that permits more precise regulation and control of the spray-freezing operation than has heretofore been obtainable. It is still another object of the invention to provide a spray-freezing method and apparatus for producing spray-frozen droplets which upon vacuum sublimation of the solvent yield solute particles of exceptionally small size. It is a still further object of the invention to provide a spray-freezing apparatus which uses a liquid freezing medium ane provides for a more efficient utilization of the freezing medium and effective separation of the spray-frozen droplets therefrom. Other objects and advantages of the present invention will be in part obvious and in part apparent from the following discussion and accompanying drawings which illustrate a preferred embodiment and a modification of the invention capable of being used in carrying out the method of the invention.

In the drawings:

FIG. 1 is a vertical section, partly in elevation, through a spray-freezing apparatus in accordance with the present invention, showing the freezing surface of the weir and a basket for collecting the mixture of coolant and ice crystals formed;

FIG. 2 is an enlarged vertical section, partly in elevation, of the spray injector assembly shown in FIG. 1;

FIG. 3 is a detailed section of a portion of the apparatus of FIG. 1 showing the support for the basket for separating the frozen solution crystals from the coolant;

FIG. 4 is a vertical section of another embodiment of the invention wherein the weir has a flat freezing surface; and,

FIG. 5 is a front view of the weir shown in FIG. 4.

Referring first to FIG. 1, the freezing apparatus as shown comprises a vertically positioned cylindrical tank 10 in which a mixing weir 12 is centrally located. Mixing weir 12 is of generally cylindrical configuration and coaxial with tank 10. It flares outward elliptically at its upper end 13 until the top edge of its wall is essentially horizontal. This upper end 13 rests on and is attached to an upper annular flange 14 that extends from the top of an inner cylindrical wall 16. Wall 16, of slightly less diameter than tank 10, is suppoted at its lower end by a lower annular flange 18 that extends from the interior surface of tank 10. The tank 10 is supported by four evenly spaced tank support legs 20, only one of which is shown in FIG. 1. The top of tank 10 is enclosed with a tank lid 22 that is circumferentially supported by an external upper tank flange 24. A sight port 26 is provided in tank lid 22 to permit operators of the apparatus to view the spray-freezing process.

An injector assembly 28, shown in detail in FIG. 2, is positioned through the center of tank lid 22 so that a spray nozzle 30 extends below the top edge of weir 12. The spray nozzle 30 is attached to the lower end of an injectorassembly pipe 31. Pipe 31 is enclosed in a water jacket 32 through which hot water flows to prevent precrystallization of the solution. Hot water enters water jacket 32 through entrance 34 and flows downwardly into conduit 35 adjacent to pipe 31. Returning water then flows upwardly in an outside annular conduit 36 and leaves water jacket at exit 37. A portion of water jacket 32 is enclosed in a sleeve 38, preferably of Teflon, and the entire assembly is held in lid 22 by a mounting cylinder 40 that is attached to the top of lid 22.

The solution to be crystallized is contained in a solution storage tank 42 that is pressurized by gas stored in a pressurization tank 44. The conduit 45, between tanks 44 and 42, contains a pressurization valve 46 that is used to control the pressurization of tank 42. A solution flow conduit 48 connects tank 42 to pipe 31 of the injector assembly 28. Adjustment of solution flow rate is controlled by a solution flow valve 50 that is located in conduit 48.

An upper heat exchanger 52 is located between the inner cylindrical wall 16 and the wall of tank 10. This heat exchanger consists of an upper refrigeration coil 54 that is supplied with a refrigerant through a refrigerant input conduit 56. The refrigerant is cooled by a refrigeration unit 58 that supplies conduit 56. The bottom turn of coil 54 is connected to the bottom turn of a lower refrigeration coil 60 by a refrigeration coil connecting conduit 62. Coil 60 is positioned in the lower portion 64 of tank and is covered by the coolant liquid that is maintained at a level 66 above the coil as shown in FIG. 1. The uppermost turn of coil 60 is connected to a refrigerant return conduit 68 that directs the flow of the refrigerant back to refrigeration unit 58.

A coolant exit port 70 is located at the bottom of tank portion 64 and is in communication with a coolant return conduit 72. A cryogenic ball valve 74 is located in conduit 72 to provide appropriate control of the coolant flow. A second cryogenic ball valve 76 is connected to the coolant drain 78 that branches from a T- connection 79 in conduit 72. Conduit 72 is connected to the input of a coolant circulating pump 80 that directs the coolant through a coolant feed conduit 82 to the bottom of heat exchanger 52. The coolant then flows upwardly around the coil 54 which serves to further refrigerate it, and flows over the upper end 13 of weir 12, forming a thin film on the inside surface of the weir. As the coolant film moves down, it is struck by the spray emitted from nozzle 30. This contact with the coolant immediately freezes the spray droplets thereby forming a slurry of the frozen droplets in the coolant. This slurry then continues to flow down the inner surface of weir l2 and drops into a basket 86 positioned in a basket support 88. Basket 86 is formed from a tightly woven screen, for example, 325 mesh, which holds the frozen droplets but permits passage of the coolant to the lower portion 64 of tank 10. The basket support 88 extends through the wall of tank 10 and an insulation and vapor barrier 90, which encloses the entire tank as well as the external various refrigerant and coolant conduits, for easy removal of basket 86. The details of this basket support extension are shown in FIG. 3, representing a partial side view of the embodiment in FIG. 1. A horizontal basket support cylinder 92, of slightly larger diameter than support 88, is joined to the front of tank 10. The outer end of support 88 is attached to an annular flange 94, which in turn is welded to the outer edge of cylinder 92. The internal diameter of flange 94 is such that basket 86 can be easily withdrawn from the interior of tank 10. During the freezing process, an insulated cover 96 closes the end of the basket support extension.

In beginning the freezing process, it is first necessary to establish a constant flow of the coolant over the upper circumferential edge of weir 12. The sight port 26 can be used to view this flow to ensure that the coolant only forms a thin film on the inner surface of the weir. As previously indicated, the coolant flows down the weir and is collected in the bottom portion 64 of tank 10, where it is refrigerated by the refrigerant flowing in coil 60. It should be noted that the flow pattern in coil 60 of the refrigerant starts in the bottom conduit and spirals upwardly to exit from the top conduit. This arrangement progressively lowers the coolant temperature as the coolant moves towards the bottom of tank 10. Portions of the coolant near the bottom of tank 10 are then removed and pumped to the upper heat exchanger 52. Since the upper refrigeration coils 54 spiral downwardly from the refrigerant input conduit 56, the passage of the coolant upward in the heat exchanger 52 progressively lowers the coolant temperature until it approaches the temperature of the incoming refrigerant.

The interior of tank 10 is maintained at such a low temperature that there is danger of precrystallization of the solution while still in the injector assembly 28. The hot water jacket 32 is therefore installed about the injector assembly to provide means for maintaining the temperature of the solution above its freezing point prior to emission from nozzle 30. Further isolation of the injector assembly and waterjacket from the tank lid 22 is provided by the sleeve 38, which is preferably constructed of Teflon or other similar material. This sleeve also protects the assembly from metal fatigue caused by temperature cycling andtemperature gradiants.

Referring now to FIG. 4 wherein a second embodiment of a freezing appartus is shown. This embodiment comprises a cylindrical tank 100 which is enclosed at opposite ends by an upper tank cover 102 and a lower tank cover 104. An outer housing 106 begins slightly below the upper tank cover 102 and encloses the remainder of tank 100. Two annular flange separators 108 maintain the lateral position of tank 100 with respect to outer housing 106. Several holes 110 in the flange separators 108 provide means for venting an evaporated cooling medium which will be discussed later. Tank 100 is supported vertically in outer housing 106 by tank support legs 112 which are attached to the bottom 114 of outer housing 106. in turn, the outer housing is supported by legs 116.

A flat mixing weir plate 1 18 is shown as part of a weir assembly 119 formed by the weir plate 118, a back plate 120, and sides 122. Weir plate 118 and back plate 120 are positioned parallel and separated so that the space between them constitutes a weir conduit 124. The upper edge of weir plate 118 is beveled at about 45 sloping downwardly towards the center of tank 100. The weir assembly 119 is supported in tank 100 by weir support bolts 128 at an angle of approximately 5 from the vertical.

A nozzle 130, forming part of a nozzle assembly 32, is positioned opposite the front portion of weir plate 1 18 such that the center axis of the nozzle is perpendicular to the front surface of plate 1 18. The nozzle assembly 132 is mounted in tank 100 on a nozzle assembly support 134 which extends between the opposite sides of the tank walls. The solution to be crystallized is contained in a solution storage tank 136 which is pressurized by gas stored in a pressurization tank 138. The conduit 140, between tanks 138 and 136, contains a pressurization valve 142 used to control the pressurization of tank 136. A solution flow conduit 144 connects tank 136 to the nozzle assembly 132. Adjustment of solution flow rate is controlled by a solution flow valve 146 located in conduit 48.

The lower portion 148 of outer housing 106 is of a larger diameter than the upper portion and constitutes a heat exchange section of the apparatus. A series of hinged inlet ports 150 are positioned along the horizontal top of lower housing portion 148 to provide means whereby a cooling medium may be inserted in the heat exchanger. A drain 152 is located on the bottom 114 of the outer housing to provide an exit for the liquefied cooling medium. As previously mentioned, holes 110 in flange separators 108 provide conduit means whereby evaporated portions of the cooling medium may rise and exit through an exhaust vent 154. Several turns of a coolant coil 156 surround the lower portion of tank 100 to form a portion of the heat exchanger. The uppermost turn of coil 156 is connected to the bottom of weir conduit 124 by conduit 158. Coolant flows through coils 156 and up through conduits 158 and 124 exiting over the upper beveled edge 126 of weir plate 118. The coolant then flows in a thin film down the sharp incline on the front surface of weir plate 118 whereupon droplets of the solution are sprayed against it and are immediately frozen thereby forming a slurry frozen droplets of the solution in the coolant. The slurry then continues to flow down to the bottom of the weir and drops into the top of a collector tube 160 which supports a collector basket 162 formed of fine screen mesh. The crystallized solution particles are separated from the coolant by the screen in basket 162 and the coolant continues to flow to the bottom of tank 100 where it exits through port 164 to the coolant return conduit 166. Conduit 166 is connected to the input of a coolant circulating pump 168 which pumps the coolant through a conduit 170 back to coils 156.

After a sufficient quantity of solution has been frozen, the process is discontinued and cover 102 is removed to permit the apparatus operator to retrieve basket 162.

In the front view of the weir assembly 119, shown in essential that any liquid, selected as a coolant, be a nonsolvent for the particular solution utilized. In cases where ammonium perchlorate is the desired product and an aqueous solution thereof is used as the feed to the spray, Freon 12 has proven to be a preferable coolant although other coolants, such as n-butyl alcohol, may also be used effectively. To prevent loss of the coolant during operation of the freezing apparatus, it is desirable to construct the apparatus to withstand pressures of at least 100 psi.

A mechanical refrigeration system, as shown in the embodiment of FIG. 1, may also be used with the embodiment of FIG. 4, provided a suitable heat exchanger is utilized. A refrigerant that may be used effectively with such a refrigeration system is Freon H3. The refrigeration unit must have sufficient cooling capacity to lower the temperature of the coolant to a point where it can crystallize the sprayed solution rapidly. It has been found that a 15 ton refrigeration unit is sufficient for an apparatus as shown in FIG. 1 where the diameter of tank 10 is 16 inches and the diameter of the cylindrical portion of weir 12 is 5 inches. In the embodiment of FIG. 4, a refrigerant such as a mixture of methylene chloride and dry ice may be packed against the coolant coils 156 in the lower portion 148 of the outer housing 106.

Similar nozzle pressurization systems have been shown in the embodiments. Preferably nitrogen is used as the pressurization gas in the pressurization tanks. It has been found desirable to maintain a pressure of at least 1,000 psig in the pressurization tanks to ensure proper spray emission from the nozzles. It will of course be understood that while only one nozzle and one weir have been shown in the described embodiments, multiple nozzles can be used with a single weir and more than one weir can be used in a single freezing apparatus.

Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effective therein by one skilled in the art without departing from the scope or spirit of this invention.

I claim:

1. An appartus for forming fine frozen crystals of a solution by using a refrigerated liquid coolant which is a non-solvent for said solution comprising in combination, a stationary coolant tube having an upper and lower end, means for supplying said coolant to the upper end of said coolant tube in an amount to form a thin downwardly flowing film of said coolant on the inner surface of said tube, at least one spray nozzle located adjacent to said tube and positioned to spray droplets of said solution against the thin downwardly flowing film of said coolant, means for supplying said solution under pressure to said spray nozzle, whereby the spray droplets of said solution are frozen by said coolant and form a slurry with said coolant which flows from the lower end of said coolant plate, and means for progressively lowering the temperature of said coolant prior to supplying said coolant to the upper end of said coolant tube comprising a refrigerant and a coiled conduit in external contact with said coolant for conducting said refrigerant in a flow direction opposite to the flow direction of the adjacent coolant.

2. An apparatus as described in claim 1 wherein, said means for progressively lowering the temperature of said coolant includes a refrigerant enclosure and coiled conduit means for conducting said coolant within said refrigerant enclosure.

3. An apparatus as described in claim 1 wherein, said coolant tube has a cylindrical-shaped lower portion, and a downwardly converging convexly curved upper portion merging with said lower portion to form a smooth interior surface on which the coolant film is established.

4. An apparatus as described in claim 1 wherein, said means for supplying said coolant comprises, conduit means ending at the upper end of said coolant plate, regulating means for controlling the coolant flow rate in said conduit means, and pumping means for moving said coolant through said conduit means.

5. An apparatus as described in claim 1 including, means for separating said frozen crystals of solution from said coolant comprising a removable basket of fine mesh screen positioned below said coolant tube.

6. An apparatus as described in claim 1 including, means for heating said solution prior to spraying from said nozzle to prevent precrystallization of said solution in said nozzle.

7. Apparatus as described in claim 1 wherein said spray nozzle is located within said coolant tube.

8. An apparatus for forming fine frozen crystals of a solution by using a refrigerated liquid coolant which is a non-solvent for said solution comprising a housing, a stationary coolant tube having an upper and lower end mounted in said housing, a cylindrical partition positioned between said housing and tube and cooperating with the wall of said housing to define a passage for conducting coolant to the upper end of said coolant tube to cause said coolant to flow downwardly in a thin film on the inner surface of said coolant tube, means for supplying coolant to said passage and spray means positioned within the upper end of said coolant tube to spray droplets of said solution against said coolant film.

9. Apparatus according to claim 8 wherein said coolant tube has a cylindrical lower portion and a downwardly converging, convexly curved upper portion merging with said lower portion to form a smooth interior surface on which the coolant film is established.

10. Apparatus according to claim 8 including a refrigerant coil mounted in said passage and means for supplying refrigerant to the upper end of said coil to cause refrigerant to flow through said coil in a direction generally counter-current to the flow of coolant through said passage. 

1. An appartus for forming fine frozen crystals of a solution by using a refrigerated liquid coolant which is a non-solvent for said solution comprising in combination, a stationary coolant tube having an upper and lower end, means for supplying said coolant to the upper end of said coolant tube in an amount to form a thin downwardly flowing film of said coolant on the inner surface of said tube, at least one spray nozzle located adjacent to said tube and positioned to spray droplets of said solution against the thin downwardly flowing film of said coolant, means for supplying said solution under pressure to said spray nozzle, whereby the spray droplets of said solution are frozen by said coolant and form a slurry with said coolant which flows from the lower end of said coolant plate, and means for progressively lowering the temperature of said coolant prior to supplying said coolant to the upper end of said coolant tube comprising a refrigerant and a coiled conduit in external contact with said coolant for conducting said refrigerant in a flow direction opposite to the flow direction of the adjacent coolant.
 2. An apparatus as described in claim 1 wherein, said means for progressively lowering the temperature of said coolant includes a refrigerant enclosure and coiled conduit means for conducting said coolant within said refrigerant enclosure.
 3. An apparatus as described in claim 1 wherein, said coolant tube has a cylindrical-shaped lower portion, and a downwardly converging convexly curved upper portion merging with said lower portion to form a smooth interior surface on which the coolant film is established.
 4. An apparatus as described in claim 1 wherein, said means for supplying said coolant comprises, conduit means ending at the upper end of said coolant plate, regulating means for controlling the coolant flow rate in said conduit means, and pumping means for moving said coolant through said conduit means.
 5. An apparatus as described in claim 1 including, means for separating said frozen crystals of solution from said coolant comprising a removable basket of fine mesh screen positioned below said coolant tube.
 6. An apparatus as described in claim 1 including, means for heating said solution prior to spraying from said nozzle to prevent precrystallization of said solution in said nozzle.
 7. Apparatus as described in claim 1 wherein said spray nozzle is located within said coolant tube.
 8. An apparatus for forming fine frozen crystals of a solution by using a refrigerated liquid coolant which is a non-solvent for said solution comprising a housing, a stationary coolant tube having an upper and lower end mounted in said housing, a cylindrical partition positioned between said housing and tube and cooperating with the wall of said housing to define a passage for conducting coolant to the upper end of said coolant tube to cause said coolant to flow downwardly in a thin film on the inner surface of said coolant tube, means for supplying coolant to said passage and spray means positioned within the upper end of said coolant tube to spray droplets of said solution against said coolant film.
 9. Apparatus according to claim 8 wherein said coolant tube has a cylindrical lower portion and a downwardly converging, convexly curved upper portion merging with said lower portion to form a smooth interior surface on which the coolant film is established.
 10. Apparatus according to claim 8 including a refrigerant coil mounted in said passage and means for supplying refrigerant to the upper end of said coil to cause refrigerant to flow through said coiL in a direction generally counter-current to the flow of coolant through said passage. 